Aircraft Batteries and Emergency Power

Types of Aircraft Batteries

Aircraft use rechargeable (secondary) batteries, mainly:

• Lead-Acid: Common in smaller aircraft; reliable but heavy and sensitive to temperature.
• Nickel-Cadmium (NiCd): Favoured in larger and transport aircraft for better performance and tolerance to deep discharge.
• Lithium-Ion/Lithium-Polymer: Increasingly used in modern designs for high energy density and lighter weight, but with significant safety risks.

Battery Functions and Limitations

The primary role of an aircraft battery is to supply electrical power when generators or external sources are unavailable—during engine/APU start or total power loss in flight. Batteries power vital and essential services, such as flight instruments, avionics, and emergency systems, through dedicated busbars. However, battery-only operation is strictly time-limited, typically around 30 minutes, depending on capacity and load.

Emergency Power and Distribution

When all generators fail, batteries supply DC power directly to the vital busbar and, via static inverters, can provide AC power to essential services. Non-essential loads are isolated to conserve battery life. The battery busbar (often called the "hot" or "direct" busbar) ensures uninterrupted supply to critical systems like fire extinguishers and emergency lighting.

Battery Monitoring and Failure

Continuous monitoring of battery voltage, temperature, and charging status is required. Overheating, overcharging, or physical damage can lead to battery failure—most critically, thermal runaway, particularly in lithium-based batteries. Thermal runaway can cause intense fires, as heat lowers internal resistance, accelerating the reaction and potentially affecting adjacent cells.

Lithium Battery Hazards and Containment

Lithium batteries pose heightened fire risks, especially if damaged, improperly charged, or exposed to high temperatures. The risk increases with the number of batteries on board, including those carried by passengers. If thermal runaway occurs in flight, containment is challenging due to the rapid escalation and potential for fire to spread between cells. Aircraft are equipped with warning systems and containment procedures, but the safest response is to land as soon as possible.

Recognising Battery Failure

Symptoms include warning indications ("battery hot"), loss of electrical services, or abnormal smells/smoke. Crews must be prepared to manage limited electrical power, prioritising essential systems and following emergency checklists.